THE Q-ANOR DIAGRAM: A TOOL FOR THE PETROGENETIC AND TECTONOMAGMATIC CHARACTERIZATION OF GRANITIC SUITES

The Q-ANOR diagram is an empirically derived CIPW normative based equivalent to the IUGS modal QAPF classification scheme for granitoids (Streckeisen and LeMaitre, 1979: Neus. Jahrb. Mineral. Abh. 136, 169-206). It employs the parameters Q= 100*(Qtz/(Qtz+Ab+Or+An)) and ANOR= (100*An/(An+Or)). In contrast to results obtained from normative triangular classification diagrams, plutonic rock type names derived from the Q-ANOR diagram closely approximate modal-derived results. Trends have been added to this diagram based on published datasets from the Peninsular Ranges (calcic: C), Tuolumne (calc-alkalic: CA), Sherman (alkali-calcic: AC), and Bjerkreim (alkalic: A) batholiths. These same major element datasets were employed to constrain the positions of the C-CA, CA-AC and AC-A suite boundaries on the widely employed SiO₂ versus Na₂O+K₂O-CaO (or modified alkali-lime index, MALI) granitic classification diagram (Frost et al., 2001: J. Petrol. 42, 2033-2048). The modified Q-ANOR plot compliments the MALI diagram by identifying rock types comprising a suite and their relative abundances.

Previous workers have employed the Q-ANOR diagram to compare Proterozoic granitic suites of ‘unknown’ tectonic context to published datasets from Paleozoic suites or batholiths formed within ‘known’ tectonic settings, thus demonstrating its value in distinguishing between a subduction zone versus nonsubduction-zone related petrogenesis (e.g. Kerr, 1989: Prec. Res. 45, 1-17; Chako et al. 2000: Can. J. Ear. Sci 37, 1597-1609). Combining this empirical comparison approach with the Frost et al. granitic classification scheme, the modified Q-ANOR diagram can help constrain the petrogenesis and tectonic environments of plutonic suites that frequently represent voluminous map units within Archean through to Paleozoic orogenic belts. This major element based approach avoids some problems associated with trace element based tectonomagmatic classification schemes, particularly the fact that many trace elements are not incompatible in granitic rocks but reside in minor silicate and oxide phases. Trace element abundances are more strongly affected by crustal assimilation and the crystallization history of the magma, whereas major elements more reliably reflect the composition of the parent magma and its source(s).